In a wireless communication system, it is necessary to synchronize timing and frequency offsets between the transmitting and receiving devices in order for the receiving device to perform data demodulation. Timing and frequency synchronization is commonly achieved by the transmission of known synchronization signals from the transmitting device and the detection of the synchronization signal by the receiving device. The synchronization signal should preferably have characteristics that are suitable for accurate detection of timing and frequency offsets resulting from the location of the receiving device, propagation channel and receiver oscillator offset.
In Long Term Evolution (LTE) systems, the base station transmits two synchronization signals on the downlink: the Primary Synchronization Signal (PSS) and the Secondary Synchronization Signal (SSS). Joint timing and frequency synchronization is performed using the PSS and SSS to obtain estimates of the timing and frequency offsets. Denoting the timing offset as τ and the frequency offset as v, the joint timing and frequency estimator searches for the pair (τ,v) that maximizes a metric. To perform a complete search, the joint timing and frequency estimator computes the metric for each possible pair (τ,v) and selects the one that maximizes the metric as its estimate of the timing and frequency offsets.
Synchronization performance can be improved by using other reference symbols transmitted by the base station in combination with the PSS and SSS for timing and frequency synchronization. For example, common reference signals (CRSs) and channel state information reference signals (CSI-RSs) transmitted by the base station can be used by an access terminal for timing and frequency synchronization along with the PSS and SSS to improve the accuracy of the timing and frequency offset estimates.
One problem encountered in performing joint timing and frequency synchronization is the complexity of the search. When the uncertainty windows for the timing and frequency offsets are large, the complexity of the search will increase and more memory and/or processing resources will be needed to perform a complete search. Compounding this problem, when different types of reference signals are combined for timing and frequency synchronization, some of the signals may increase uncertainty for the timing and/or frequency offsets and thus increase the complexity of the search.